"This is an EU-UK matter, we are the EU, there is no scope for a bilateral agreement," Neale Richmond, the Brexit spokesman for Ireland's ruling Fine Gael party, tweeted, adding that the apparent proposal was a "worrying pivot" by the UK government.

Ireland's Deputy Prime Minister Coveney told RTE Radio that Ireland would not be "steam-rolled" by Britain.

"There is a consequence to the approach that the British Government is taking and that consequence is that they are making a no-deal far more likely," he said.

The proposal for a new trade union between the two countries is reportedly one of the "creative solutions" Johnson has said he'll raise with EU leaders this week.

"There are plenty of other creative solutions" to the border issue, Johnson told ITV News on Tuesday.

"I'm going to go and see if I can explore those ideas with our friends in Germany and France, and at the G7. Let's see where we get to."

The prime minister is due to meet for dinner with German Chancellor Angel Merkel on Wednesday evening before having lunch with French President Emmanuel Macron on Thursday.

Johnson's proposal comes after he wrote to the European Council's president, Donald Tusk, this week to insist that the Northern Ireland backstop must be removed from the Brexit withdrawal agreement.

Tusk dismissed the idea and accused Johnson of secretly wanting to trigger a new hard border on the island of Ireland.

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